728 research outputs found
Stars caught in the braking stage in young Magellanic Clouds clusters
The color-magnitude diagrams of many Magellanic Cloud clusters (with ages up
to 2 billion years) display extended turnoff regions where the stars leave the
main sequence, suggesting the presence of multiple stellar populations with
ages which may differ even by hundreds million years (Mackey et al. 2008,
Milone et al. 2009, Girardi et al. 2011). A strongly debated question is
whether such an extended turnoff is instead due to populations with different
stellar rotations (Girardi et al. 2011, Goudfrooij et al. 2011, Rubele et al.
2013, Li et al. 2014). The recent discovery of a `split' main sequence in some
younger clusters (about 80--400Myr) added another piece to this puzzle. The
blue (red) side of the main sequence is consistent with slowly (rapidly)
rotating stellar models (D'Antona et al. 2015, Milone et al. 2016, Correnti et
al. 2017, Milone et al 2016), but a complete theoretical characterization of
the observed color-magnitude diagram appeared to require also an age spread
(Correnti et al. 2017). We show here that, in three clusters so far analyzed,
if the blue main sequence stars are interpreted with models that have been
always slowly rotating, they must be about 30% younger than the rest of the
cluster. If they are instead interpreted as stars initially rapidly rotating,
but that have later slowed down, the age difference disappears, and "braking"
also helps to explain the apparent age differences of the extended turnoff. The
age spreads in Magellanic Cloud clusters are a manifestation of rotational
stellar evolution. Observational tests are suggested.Comment: Accepted for publication and in state of Advance Online Publication
(from 24 July 2017) on Nature Astronom
Modeling lithium rich carbon stars in the Large Magellanic Cloud: an independent distance indicator ?
We present the first quantitative results explaining the presence in the
Large Magellanic Cloud of some asymptotic giant branch stars that share the
properties of lithium rich carbon stars. A self-consistent description of
time-dependent mixing, overshooting, and nuclear burning was required. We
identify a narrow range of masses and luminosities for this peculiar stars.
Comparison of these models with the luminosities of the few Li-rich C stars in
the Large Magellanic Cloud provides an independent distance indicator for the
LMCComment: 7 pages, 2 figure
Towards a working model for the abundance variations within Globular Clusters stars
A popular self--enrichment scenario for the formation of globular clusters
assumes that the abundance anomalies shown by the stars in many clusters are
due to a second stage of star formation occurring from the matter lost by the
winds of massive asymptotic giant branch (AGB) stars. Until today, the
modellizations of the AGB evolution by several different groups failed, for
different reasons, to account for the patterns of chemical anomalies. Here we
show that our own modelling can provide a consistent picture if we constrain
the three main parameters which regulate AGB evolution: 1) adopting a high
efficiency convection model; 2) adopting rates of mass loss with a high
dependence on the stellar luminosity; 3) assuming a very small overshooting
below the formal convective regions during the thermal pulse (TP) phase. The
first assumption is needed to obtain an efficient oxygen depletion in the AGB
envelopes, and the second one is needed to lose the whole stellar envelope
within few thermal pulses, so that the sum of CNO elements does not increase
too much, consistently with the observations. The third assumption is needed to
fully understand the sodium production. We also show that the Mg - Al
anticorrelation is explained adopting the higher limit of the NACRE rates for
proton captures by Mg25 and Mg26, and the models are consistent with the
recently discovered F-Al correlation. Problems remain to fully explain the
observed Mg isotopes ratios
Predictions for self-pollution in globular cluster stars
Full evolutionary models have been built to follow the phases of asymptotic
giant branch evolution with mass loss for metal mass fractions from Z=0.0002 to
Z=0.004. For the first time, we find that temperatures close to or even larger
than 10^8 K are achieved at low Z; the full CNO cycle operates at the base of
the envelope, the Oxygen abundance for the most metal-poor models of 4 and 5
solar masses is drastically reduced, and sodium and aluminum production by by
proton capture on neon and magnesium can occur. These results may be relevant
for the evolution of primordial massive globular clusters: we suggest that the
low-mass stars may have been polluted at the surface by accretion from the gas
that was lost from the evolving intermediate-mass stars at early ages.Comment: 14 pages, 6 figure
The Lithium test for multiple populations in Globular Clusters: Lithium in NGC 2808
In the globular cluster NGC2808, a quasi-standard initial lithium abundance
is derived for a red giant belonging to the `extreme' population, characterized
by a large helium overabundance, and by abundances of proton capture elements
typical of nuclear processing in gas at very high temperatures, where the
initial lithium has been fully destroyed. The observations of lithium in such
extreme cluster stars are important to test different models for the formation
of multiple populations in old Globular Clusters. In the asymptotic giant
branch (AGB) scenario, fresh lithium is synthetized during the initial phases
of hot bottom burning which, afterwards, synthetize the other p-capture
elements. We model the abundance of lithium in the ejecta of superAGB models,
finding values consistent or larger than observed in the `extreme' giant; these
same models describe correctly the magnesium depletion and silicon enrichment
of the extreme population of NGC 2808, so the overall agreement provides
further support to the AGB scenario. In the models involving massive or
supermassive stars, the Lithium observed requires a mixture of the lithium-free
ejecta of the polluting population with more than 40% of standard-lithium
pristine gas. The extended chemical anomalies of NGC 2808 stars are then to be
all explained within at most 60% of the possible dilution range, the initial
helium mass fraction in the ejecta should be Y >= 0.5, to account for the Ye
0.38-0.40 of the extreme population, and further observations of p-process
elements are needed to check the model.Comment: accepted for publication in The Astrophysical Journal Letter
Rapidly rotating second-generation progenitors for the blue hook stars of {\omega} Cen
Horizontal Branch stars belong to an advanced stage in the evolution of the
oldest stellar galactic population, occurring either as field halo stars or
grouped in globular clusters. The discovery of multiple populations in these
clusters, that were previously believed to have single populations gave rise to
the currently accepted theory that the hottest horizontal branch members (the
blue hook stars, which had late helium-core flash ignition, followed by deep
mixing) are the progeny of a helium-rich "second generation" of stars. It is
not known why such a supposedly rare event (a late flash followed by mixing) is
so common that the blue hook of {\omega} Cen contains \sim 30% of horizontal
branch stars 10 , or why the blue hook luminosity range in this massive cluster
cannot be reproduced by models. Here we report that the presence of helium core
masses up to \sim 0.04 solar masses larger than the core mass resulting from
evolution is required to solve the luminosity range problem. We model this by
taking into account the dispersion in rotation rates achieved by the
progenitors, whose premain sequence accretion disc suffered an early disruption
in the dense environment of the cluster's central regions where
second-generation stars form. Rotation may also account for frequent
late-flash-mixing events in massive globular clusters.Comment: 44 pages, 8 figures, 2 tables in Nature, online june 22, 201
Star Formation in the Starburst Cluster in NGC 3603
We have used new, deep, visible and near infrared observations of the compact
starburst cluster in the giant HII region NGC 3603 and its surroundings with
the WFC3 on HST and HAWK-I on the VLT to study in detail the physical
properties of its intermediate mass (~ 1 - 3 M_sun) stellar population. We show
that after correction for differential extinction and actively accreting stars,
and the study of field star contamination, strong evidence remains for a
continuous spread in the ages of pre-main sequence stars in the range ~ 2 to ~
30 Myr within the temporal resolution available. Existing differences among
presently available theoretical models account for the largest possible
variation in shape of the measured age histograms within these limits. We also
find that this isochronal age spread in the near infrared and visible
Colour-Magnitude Diagrams cannot be reproduced by any other presently known
source of astrophysical or instrumental scatter that could mimic the luminosity
spread seen in our observations except, possibly, episodic accretion. The
measured age spread and the stellar spatial distribution in the cluster are
consistent with the hypothesis that star formation started at least 20-30 Myrs
ago progressing slowly but continuously up to at least a few million years ago.
All the stars in the considered mass range are distributed in a flattened
oblate spheroidal pattern with the major axis oriented in an approximate
South-East - North-West direction, and with the length of the equatorial axis
decreasing with increasing age. This asymmetry is most likely due to the fact
that star formation occurred along a filament of gas and dust in the natal
molecular cloud oriented locally in this direction.Comment: 21 pages, 19 figures, accepted for publication in Astrophysics &
Space Scienc
The CFHT Open Star Cluster Survey. IV. Two Rich, Young Open Star Clusters: NGC 2168 (M35) and NGC 2323 (M50)
We continue our study of rich Galactic clusters by presenting deep CCD
observations of both NGC 2168 (M35) and NGC 2323 (M50). Both clusters are found
to be rich (NGC 2168 contains at least 1000 stars brighter than V = 22 and NGC
2323 contains approximately 2100 stars brighter than our photometric limit of V
= 23) and young (age of NGC 2168 = 180 Myrs, age of NGC 2323 = 130 Myrs). The
color-magnitude diagrams for the clusters exhibit clear main sequences
stretching over 14 magnitudes in the V, B-V plane. Comparing these long main
sequences with those of earlier clusters in the survey, as well as with the
Hyades, has allowed for accurate distances to be established for each cluster
(dist. of NGC 2168 = 912 +/- 70/65 pc, dist. of NGC 2323 = 1000 +/- 81/75 pc).
Analysis of the luminosity and mass functions suggest that despite their young
ages, both clusters are somewhat dynamically relaxed exhibiting signs of
mass-segregation. This is especially interesting in the case of NGC 2323, which
has an age of only 1.3 times the dynamical relaxation time. The present
photometry is also deep enough to detect all of the white dwarfs in both
clusters. We discuss some interesting candidates which may be the remnants of
quite massive (M > 5 Mo) progenitor stars. The white dwarf cooling age of NGC
2168 is found to be in good agreement with the main-sequence turn-off age.
These objects are potentially very important for setting constraints on the
white dwarf initial-final mass relationship and upper mass limit for white
dwarf production.Comment: 34 pages, including 12 diagrams and 5 tables. Accepted for
publication in AJ. Minor typos correcte
The Nearby Evolved Stars Survey I. JCMT/SCUBA-2 Submillimeter Detection of the Detached Shell of U Antliae
We present the highest resolution single-dish submillimetre observations of the detached shell source U Antliae to date. The observations were obtained at 450 and 850m with SCUBA-2 instrument on the James Clerk Maxwell Telescope as part of the Nearby Evolved Stars Survey. The emission at 850m peaks at 40 arcsec with hints of a second peak seen at 20 arcsec. The emission can be traced out to a radius of 56 arcsec at a 3 level. The outer peak observed at 850m aligns well with the peak observed at Herschel/PACS wavelengths. With the help of spectral energy distribution fitting and radiative transfer calculations of multiple-shell models for the circumstellar envelope, we explore the various shell structures and the variation of grain sizes along the in the circumstellar envelope. We determine a total shell dust mass of (2.0 0.3) 105 M and established that the thermal pulse that gave rise to the detached shell occurred 3500 500 yr ago
Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars
Red giants are evolved stars that have exhausted the supply of hydrogen in
their cores and instead burn hydrogen in a surrounding shell. Once a red giant
is sufficiently evolved, the helium in the core also undergoes fusion.
Outstanding issues in our understanding of red giants include uncertainties in
the amount of mass lost at the surface before helium ignition and the amount of
internal mixing from rotation and other processes. Progress is hampered by our
inability to distinguish between red giants burning helium in the core and
those still only burning hydrogen in a shell. Asteroseismology offers a way
forward, being a powerful tool for probing the internal structures of stars
using their natural oscillation frequencies. Here we report observations of
gravity-mode period spacings in red giants that permit a distinction between
evolutionary stages to be made. We use high-precision photometry obtained with
the Kepler spacecraft over more than a year to measure oscillations in several
hundred red giants. We find many stars whose dipole modes show sequences with
approximately regular period spacings. These stars fall into two clear groups,
allowing us to distinguish unambiguously between hydrogen-shell-burning stars
(period spacing mostly about 50 seconds) and those that are also burning helium
(period spacing about 100 to 300 seconds).Comment: to appear as a Letter to Natur
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